To modify the input array, pass an array reference to the subroutine.

#!usr/bin/perl -w
use strict;

my @arr = qw( 1 2 3 4 5 6 7 8); 
square(\@arr);

sub square
{
   my $arrayref = shift;
   foreach my $item (@$arrayref)
   {
      $item **=2; #or
     #$item *=$item;
   }
}

foreach (@arr)
{
   print "$_\n";
}
__END__
prints:
1
4
9
16
25
36
49
64
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This will produce the same @arr, but not quite in the same way:

my @arr = qw( 1 2 3 4 5 6 7 8); 
foreach (@arr)
{
  $_*=$_;
}
#or 
@arr = map($_*$_}@arr;
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